A lithium secondary battery is fabricated by assembling a cathode material, an anode material, an electrolyte, and a separation film, and does not use heavy metal, such as mercury (Hg) or cadmium (Cd), so that the lithium secondary battery is environmentally friendly, has a higher output voltage than that of an existing battery, and has a large capacity. Owing to a characteristic of a light lithium atom, the lithium secondary battery is widely used in a superlight electronic product, such as a mobile phone and a camcorder, and recently, the use of the lithium secondary battery is expanded to an electric vehicle and a usage, such as energy storage, according to the development of a large capacity technology.
The lithium secondary battery is divided into a general lithium ion battery which uses a liquid electrolyte and a lithium ion polymer battery which uses an electrolyte in a form of gel or a solid polymer according to the kind of electrolyte. Further, the lithium secondary battery is divided into a cylindrical shape and a square shape according to a shape of the battery.
A lithium ion polymer battery adopting a method of using a completely solid-type electrolyte among the lithium ion polymer batteries is not generally used well because of low ion conductivity of the electrolyte.
The lithium ion polymer battery does not have an electrolyte leakage problem or has a simple method of preventing a leakage of the electrolyte, compared to a general lithium ion battery using a liquid electrolyte. The lithium ion polymer battery uses a metal can or has a pouch form formed with multiple layers of metal foil and a polymer, thereby preventing an electrolyte from leaking.
When a pouch is used as a battery case, the lithium ion polymer battery has an advantage in that it is possible to remarkably decrease weight of the battery compared to the case where the metal can is used. Aluminum is generally used as a metal for foil forming an exterior layer of the pouch. A polymer layer forming an interior layer of the pouch is mainly formed of polypropylene, which protects the metal foil from the electrolyte and prevents a cathode, an anode, and electrode taps from being short-circuited.
In the lithium secondary battery, lithium ions (Li+) move from a cathode to an anode during discharging, and lithium ions (Li+) move from the anode to the cathode during charging, so that the lithium secondary battery accompanies an electrochemical reaction inside the battery. According to the electrochemical reaction, the lithium secondary battery is repeatedly contracted and expanded, and discharges gas.
Further, in the secondary battery, when an overcurrent flows by overdischarging or a short-circuit, a temperature inside the battery increases and a phase change is generated from the liquid electrolyte to gas, and for this reason, a pressure inside the battery increases.
In the case of the pouch-type lithium secondary battery, it is possible to decrease weight of the battery as described above, but the pouch-type lithium secondary battery has a problem in that strength of the foil-type metal layer forming the exterior layer of the pouch is weak, so that it is impossible to sufficiently endure a pressure of the internal gas. Accordingly, when the battery is expanded by gas generated inside the battery, performance of the battery deteriorates and as a result, a life of the battery is decreased.
Particularly, a lithium cobalt oxide (LiCoO2) currently widely used in the lithium secondary battery has an advantage of high capacity and output, but has a characteristic that a material structure is unstable during charging or discharging, so that when an overcurrent flows by a short-circuit and the like, this may lead to an explosion and a fire.
In order to solve the problem due to the generation of the gas inside the lithium secondary battery, various research has been currently conducted. However, in the existing research, only a means for discharging gas inside the battery is simply provided, or a strain gauge is directly mounted in an electrode assembly, so that a thickness of the battery is increased.
Accordingly, research for maintaining the advantages of the pouch-type secondary battery, efficiently removing gas generated inside the battery, and minimizing a battery cell swelling phenomenon and controlling charging by detecting a deformation of a battery case due to the gas generated inside the battery at an early stage has been emphasized than ever.